Wirebonding is a widely used technique for electrically connecting contacts within a semiconductor package. Commonly, a precious metal wire, normally gold within the range of approximately 0.0010 and 0.0015 inches in diameter, has one end ball-bonded to a bond pad on an IC, and another end stitch (or wedge) bonded to a lead on a leadframe. In order to accomplish this, the wire is fed through a capillary associated with a moveable bond head. For a ball bond, a ball is formed on the exposed end of the wire using an electronic flame off (EFO) mechanism. The ball is pulled against the end of the capillary and is then pressed into position on a pre-heated bond pad where a combination of heat, pressure, and ultrasonic vibration is used to cause the ball to adhere to the surface of the bond pad. With the ball end of the wire secured to the bond pad, the gold wire is payed out through the capillary as the bond head moves into position at the appropriate lead. A stitch bond is formed on the lead, and a tail wire is payed out through the capillary, clamped, and severed. A new ball is then formed readying the wire end for the next ball bond, and the cycle is repeated.
Bond head machinery typically includes a fixed ultrasonic horn, which includes the capillary and is equipped for movement along the z-axis. Various peripheral mechanical and electronic systems support the implementation of the general wirebonding procedure described. Associated wire handling machinery typically includes a spool, tensioner, clamp, ball detector, and gas-powered venturi for feeding wire to the capillary. Movement in the x- and y-axes is implemented primarily by moving the bond head assembly in order to position the horn over the bond target.
In efforts to overcome various problems in the arts, dual bond head systems and techniques are sometimes used. In some applications, for example, two separate bond heads are oriented for making bonds perpendicular to one another. In such a dual bond head configuration, each bond head independently performs wirebonding with a capillary on a horn in a fixed orientation. Supposing, for instance, that a 20-pin IC required fourteen bonds oriented in one direction and six oriented in a direction perpendicular relative to the others. Using a dual head bonder known in the arts, each head may perform the first six of its bond wire installations simultaneously. One bond head would then be idled, while the other completed the remaining, in this case eight, bond wires. Such workload imbalances are relatively common among dual head bonding systems and methods using apparatus known in the arts. Thus, inefficiency is a problem with the dual bond head approach current in the art.
A significant portion of IC package manufacturing costs is due to the expense of precious metals, thus there are ongoing efforts to reduce the precious metal content of IC packages. Many IC package applications may be characterized as having at least two clearly separable groups of bond wires that, in theory, may employ significantly different wire gauges and still maintain reliable function. For example, a buffer function is readily separable into supply and output stages with high current demands, while input stages operate at current demands that are very small in comparison. Despite the opportunity for precious metal reduction by using smaller wire gauges on the input side, predominant applications commonly deploy the same wire size throughout an assembly, using the wire gauge demanded by the worst-case current path. Previous attempts to reduce gold content by use of dual head tandem arrangements have found little acceptance because such arrangements tend to reduce overall throughput due to the workload imbalance between the heads.
Due to these and other technological problems, improved apparatus and methods for wirebonding with more than one wire source would be useful and advantageous contributions to the art. The present invention is directed to overcoming, or at least reducing, problems present in the prior art, and also contributes one or more heretofore unforeseen advantages indicated herein.